This invention relates generally to apparatus for heat treating particulate material. More particularly, the instant invention relates to a method and apparatus for cooking, roasting or drying a particulate food product by exposure to a granular bed of heat transferring material.
In the prior art it is well known to treat particulate products by mixing the product with a heated granular material such as salt, sand, metal pellets or the like. Products such as beans, coffee, soybeans, cereal grains, small parts or the like are particularly well adapted for processing through granular bed processes.
In a typical prior art system salt or granular material such as sand or metal pellets are heated and then mixed with the product to be processed, whereupon heat transfer between the granular material and the product will facilitate processing. Typically, a prior art device will include a conveyor means for introducing granular material into a heating chamber, means for introducing the product to be cooked with the granular material into the heating chamber, means for drawing the commingled product through a heating chamber, and of course means for generating heat. Usually a separator is included to separate finished product from granular material, and to return granular material to the input of the heating chamber to repeat the cycle. Examples of prior art constructed in accordance with the aforementioned general guidelines are U.S. Pat. Nos. 2,872,386, issued to O. Aspegren on Feb. 3, 1959; 3,253,533, issued to J. O. Benson on May 31, 1966; and 3,746,546, issued to R. N. Bateson et al on July 17, 1973. Further known background information appears in the following articles: Raghavan and Harper, "High Temperature Drying Using a Heated Bed of Granular Salt," Transactions of the A.S.A.E., Volume 17, No. 1, Pages 108-111, 1974; Raghavan, Harper, Haberstroh, "Heat Transfer Study Using Granular Media," Transactions of the A.S.A.E., Volume 17, No. 3, Pages 589-592, 1974; and Raghavan, Harper and Kienholz, "Nutritive Value of Salt Bed Roasted Soybeans for Broiler Chicks," Poultry Science, Volume 53, Pages 547-553, 1974.
In prior art food processing devices of the type disclosed in the above-mentioned patents, heating is typically provided through the process of conduction. Bateson U.S. Pat. No. 3,746,546, for example, provides a plurality of spaced apart electric heating bands clamped around a conveyor barrel for heating the barrel and the granular contents thereof. As the outside of the conveyor chamber is elevated in temperature, cooking progresses interiorly thereof as heat is conducted internally to the granular material disposed therewithin. Benson U.S. Pat. No. 3,253,533 utilizes gas heating of the outside of a granular container pan.
While designs of the latter type have functioned adequately under some circumstances, certain problems relating to thermal efficiency and uniform processing of the product have been apparent. For example, the use of spaced apart conductive heaters is very inefficient where relatively large conveyors are employed. Since the heat transfer area of a tube is relatively small compared to the characteristic volume enclosed thereby, inefficient thermal transfer characteristics would be expected. Moreover, the use of electrical heating bands spaced around a conductive cylinder is not as economical as the use of a direct flame heating system. When, however, direct flame heating of the cylinder body is employed the production of hot spots internally of the heating chamber can result in a burned and hence useless product. In addition, many prior art devices do not expose the product to heat for a uniform time and temperature interval and accordingly some product will be properly processed while other product is inadequately processed.
A similar approach has been employed in nonanalagous prior art for processing hydrocarbons. Examples of related prior art in this area are U.S. Pat. Nos. 1,712,083, issued to H. Koppers, May 7, 1929; 2,441,386, issued to C. Berg, May 11, 1948; 2,420,376, issued to E. Johansson, May 13, 1947; 2,494,695, issued to F. Fisher, Jan. 17, 1950; and 1,899,887, issued to E. Thiele, Feb. 28, 1933. The first three of the above-mentioned references are relevant to the instant invention. Koppers U.S. Pat. No. 1,712,083, for example, mixes material to be heated with previously heated solid balls, and heats the balls in a separate furnace between a pair of spaced apart drums. Berg U.S. Pat. No. 2,441,386 discloses apparatus for separating oil from shale. Spent shale is preheated in a rotary roaster and introduced into a separate rotatable bin with fresh shale to separate and recover the oil therefrom.